In the manufacture of semiconductors, integrated circuit patterns are transferred to semiconductor wafers from photomasks by lithography processes using precision image projection tools. Over the last several years, patterns have become smaller and smaller, allowing more devices to be packed onto a chip. The ability to print intricate patterns with excellent image fidelity requires very sophisticated and expensive image projection systems. Various types of illumination systems, such as annular illumination systems, may be used to improve imagery.
Referring now to FIG. 1, there is shown a typical image projection system used in integrated circuit lithography. A light source (not shown) shines a cone of light 10 through photomask 12 having a patterned front surface 14. The light is focused by projection lens 16 onto image plane 18. Typically, a photoresist-coated wafer (not shown) on image plane 18 receives the light, exposing the photoresist and, thus, transferring the image from photomask 12 to the wafer.
It is vital that the illumination systems of such image projection systems are properly optimized, aligned, and uniform across the entire circuit pattern being projected. Otherwise, a variety of imaging imperfections can arise from problems in the illuminator, such as image asymmetry and pattern shifting due to defocus.
Thus, it is beneficial to be able to measure the illumination in a convenient and practical manner to assure proper exposure. It has been proposed in the past to study variations in linewidth (of the projected integrated circuit lines) by mapping an estimated local value of partial coherence for random points across a stepper exposure field and comparing such estimates to the designed value. This is a time-consuming method, and only gives an indication of the size, rather than a detailed picture of the illumination shape.
Another common test is a "telecentricity" test, in which out-of-focus test patterns are exposed, and shifts of these out-of-focus images reveals mis-centering of the illumination. This test only measures the center position of the illumination, however, and does not reveal any information about the size or shape of the illumination.
It has also been proposed to generate a pupil diagram, or "pupil-o-gram", to characterize the illumination, as discussed by Joseph Kirk and Christopher Progler, two of the inventors of the present invention, in Pinholes and Pupil Fills, published in Microlithography World, Autumn 1997, pp. 25-28, and incorporated herein by reference. The pupil-o-gram uses what is essentially a pinhole camera technique, wherein a small transparent spot in an opaque photomask (pinhole) or a small opaque spot in a transparent photomask (reverse pinhole) is projected onto the image plane. The light traveling through the pinhole (or around the reverse pinhole) forms a pupil-o-gram image some distance below the wafer plane (typically 10-30 mm), where it can be captured on photochromic film. The pupil-o-gram corresponds to pupil illumination 17 at pupil 19 (having aperture 21) of projection lens 16, hence the term "pupil" diagram or "pupil"-o-gram.
The pupil-o-gram can then be quantitatively evaluated to determine if the distribution of illumination corresponds to the expected results, given the projector settings. The utility of the pinhole camera technique is limited, however, by a tradeoff between contrast and resolution, resulting in the created pupil-o-gram being either high-contrast, low-resolution or low-contrast, high-resolution.
Thus, there is still a need in the art to produce a bright (high-contrast), sharp (high-resolution) image in a convenient and practical manner, to be used in evaluating the illumination source of an image projector.